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Most agree (including those of us at JMP) that flow balancing valves should be placed on the return side of coils whenever possible. Why? Because this location helps reduce air and noise problems within the system.

ASHRAE states that "water velocity noise is not caused by water but by free air, sharp pressure drops, turbulence, or a combination of these, which in turn cause cavitation or flashing of water into steam" (2009 ASHRAE Fundamentals Handbook, Chapter 22). Compared with placement on the supply side, flow balance valves on the return side will reduce the amount of free air in the coil and thus the potential for noise.

Flow Control Isn’t Compromised

At a given temperature, the amount of air in water depends on pressure. If the water pressure is reduced, air is released. This phenomenon is experienced by a diver if he quickly decreases the pressure on his body by ascending quickly to the surface. A rapid decrease in pressure will cause gases to come out of solution of blood and tissue, causing pain and possible damage. This is known as "the bends".

Flow balance valves placed on the return side will result in higher water pressures within the coil, which means that more air will remain in the solution and out of the coil (see Figures 1 and 2).

In each figure, the coil pressure drop is 20 ft. Note that the flow balancing effect of the valve is the same in either case so there is no compromise with respect to controllability. The overall circuit pressure drop is the same, which will result in the same flow through the coil.